Role of nonpolar forces in aqueous solvation: Computer simulation study ofsolvation dynamics in water following changes in solute size, shape, and charge

The coupling between solvent fluctuations and the electronic states of solu tes is critically important in charge transfer and other chemical reactions . This has piqued enormous interest in solvation dynamics-the study of how solvent motions relax changes in a solute's charge distribution. In nearly every computer simulation of solvation dynamics, the system is modeled by a n atomic or molecular solute whose charge (or higher multipole moment) is s uddenly changed, and the motions of the solvent molecules that relax the ne w charge distribution are monitored. Almost none of this work, however, acc ounts for the fact that most reacting solutes also undergo significant chan ges in size and shape as well as charge distribution. For the excited state s of dye molecules typically used as probes in solvation experiments or for the atoms and molecules that change oxidation state in charge transfer rea ctions, we expect changes in reactant size on the order of 5-20%. In this p aper, we use computer simulation to explore the differences between dielect ric solvation, due to changes in charge distribution, and mechanical solvat ion, due to changes in size and shape, for a Lennard-Jones sphere in flexib le water. The solvation energy for the size changes expected in typical rea ctions is on the same order as that for the appearance of a fundamental uni t of charge, indicating that dielectric and mechanical solvation dynamics s hould participate at comparable levels. For dielectric solvation, solvent l ibrations dominate the influence spectrum, but we also find a significant c ontribution from the water bending motion as well as low-frequency translat ions. The influence spectrum for mechanical solvation, on the other hand, c onsists solely of low-frequency intermolecular translational motions, leadi ng to mechanical solvation dynamics that are significantly slower than thei r dielectric counterparts. The spectrum of couplings for various mechanical perturbations (size, shape, or polarizability) depends somewhat on the mag nitude of the change, but all types of mechanical relaxation dynamics appea r qualitatively similar. This is due to the steepness of the solute-solvent interaction potential, which dictates that the majority of the solvation e nergy for mechanical changes comes fi-om the translational motion of the cl osest one or two solvent molecules. Finally, we explore the solvation dynam ics for combined changes in both size and charge and find that the resultin g dynamics depend sensitively on the sign and magnitude of both the size an d charge changes. For some size/charge combinations, the translational and rotational motions that lead to relaxation work cooperatively, producing ra pid solvation. For other combinations, the key translational and rotational solvent motions for relaxation are antagonistic, leading to a situation wh ere mechanical solvation becomes rate limiting: solvent rotational motions are "frustrated" until after translational relaxation has occurred. All the results are compared with previous experimental and theoretical studies of solvation dynamics, and the implications for solvent-driven chemical react ions are discussed.